Lack of emissions, lower investment costs and a lower energy requirement as a result of short drying units, high production rates by virtue of rapid curing, and in many cases enhanced quality of coating, especially with regard to gloss and abrasion resistance, are the reasons why, in the field of industrial coatings, radiation curing constitutes the application form which is showing the greatest expansion.
Radiation-curing printing inks are known and are described, for example, in "UV & EB curing formulation for printing inks, coatings & paints" (R. Holman, P. Oldring, London 1988).
Their properties are causally linked with the oligomers on which they are based. In this context, the majority of commercially available oligomers in radiation-curing (UV/EB) systems are based on modified acrylates. Principal binders (30 to 95 wt. % based on the ink) are oligoineric acrylic ester compounds based on the polyethers, polyesters, epoxy resins, or polyurethanes. The average molecular weights lie customarily in the range of 200 to 4000 g/mole. Through the addition (0 to 50 wt. %) of low-viscosity mono- or multifunctional monomers, as for example hexanediol acrylate, tripropyleneglicol diacrylate, trimethylolpropane triacrylate, and the like, which serve as reactive diluents, the process viscosity can be adjusted if necessary to the desired level.
The epoxy acrylates which are frequently used in particular for coatings on paper are highly regarded for their rapid curing and the chemical resistance and hardness which can be achieved. For high-grade coatings, urethane acrylates are also employed, giving rise to excellent wetting properties, chemical resistance and hardness.
By using polyether acrylates, in contrast, it is more readily possible to achieve a desired, lower processing viscosity. Admittedly, it is necessary in this case to accept losses in respect of the abovementioned properties.
The curing mechanism presents a radiation-induced free-radical polymerization. In cases of ultraviolet-curing, the polymerization is initiated through the photoreaction of one or more initiators (0-3 to 10 wt. %). Examples of such photoinitiators are acylphosphineoxide-, acetophenone- and benzophenone-derivatives as well as thioxanthone. For acceleration, amine derivatives are occasionally used as synergists. Additional formulation constituents are one or more pigment(s). The pigment content (0.1 to 30 wt. %) depends particularly on the actual pigment type. As further additives, aerating agents as well as dispersing aids are sometimes employed.
An important area of application, in addition to the wood-processing industry, comprises that of printing inks for paper, such inks being used, for example, for the printing of record sleeves, book covers, scenic and fine art postcards and high-grade catalogs. In the course of the industrial manufacture of these printed products, the handling of these articles presents difficulties. For instance, damage to the surface of the stacked units following radiation-induced curing of the ink cannot always be avoided.
In the manufacture of printed packaging materials, moreover, a rapid release action of the printing ink is desirable, so that labels or codes applied shortly after the printing operation can be removed again at a later point in time without damaging the printed image.
Attempts have already been made to improve the handling properties of newly printed articles by adding friction-reducing additives, such as oils or waxes (e.g. polyethylene waxes or polytetrafluoroethylene waxes), to the printing ink or applying them subsequently to the printed surfaces. In many cases this leads to a disruptive loss of gloss. Moreover, the subsequent application of wax to the printed product is not always satisfactory, especially since this additional step in the process raises the manufacturing costs. In addition, it is necessary to use high concentration in order to achieve an enhancement of scratch resistance. A significant release action is not obtained in this way.
As in air-drying systems or those which operate with forced drying (temperature), silicone oils, or else organically modified siloxanes, for example polyether-siloxanes, are nowadays also employed for these purposes. However, these compounds are not incorporated chemically into the film in the course of the radiation-induced crosslinking reaction, and as a result these additives, owing to their incompatibility, rise to the surface over time, and the silicone can, on the one hand, for example in the case of repeated printing processes, reach places where it has a disruptive effect, and, on the other hand, the effect of enhanced scratch resistance is at best temporary. In particular, in the course of stacking operations it is not possible completely to avoid the transfer of the silicone additive to the reverse side of the overlying printed product.
Moreover, in the packaging industry it must be ensured that the addition of additive provides the printed product with a release effect in as short a time as possible, so that adhesive labels or codes which are applied can be removed subsequently without damaging the printed product.
In practice there is therefore a need for crosslinkable, modified silicone additives which, in low concentrations, enhance the handling properties of printed articles, especially those printed in runs, these additives in particular enhancing the scratch resistance of the fresh surfaces, increasing their lubricity, exhibiting a high release action very rapidly after crosslinking and, owing to their crosslinking, remaining in stationary form in the film. Such additives should at the same time be substantially independent of the nature and composition of the printed ink to which they are added to enhance the abovementioned properties and should be capable of universal application. These additives should be effective in minimal quantities arid should not impair the performance properties of the printing ink. In particular, they should not adversely affect the development of the surface film and the curing of the printing ink. They must, furthermore, have no deleterious effect on the stability of the printing ink and must not impair the leveling properties.